Method and system for enhanced oil recovery using pumped liquid phase propane and liquid phase butane in an adjustable ratio

ABSTRACT

A liquid phase enhanced oil recovery system (EOR) that pumps a liquid mixture consisting mostly of propane (C3) and butane (C4) into a horizontal or vertical well during the stimulation treatment phase to recover additional hydrocarbons from the reservoir. During the subsequent production phase a liquid mixture is produced that includes water, hydrocarbon vapor, and hydrocarbon liquids which include oil and C3 and C4. The C3 and C4 is recovered from the hydrocarbon vapor and can be liquefied, combined with the liquid C3-C4 recovered from the produced liquid hydrocarbon stream and then all the C3-C4 can be recycled back into subsequent stimulation treatment phases. The method works at relatively low pressures so that a pump can be used for stimulation treatment. The site equipment is light enough that it can be mounted on one or more skids to ensure transportability to be used on other wells. 
     The deployment of the skid mounted Recycle System will reduce fugitive gas emissions that are inherent in most oil and gas production operations, thereby reducing greenhouse gas emissions associated with the existing operation. 
     The invention includes Thermal Induced Fracturing (TIF) which increases flow rate and increases stimulated rock volume (SRV) resulting in additional oil recovery above the base EOR method. The invention includes Spalling in fractures from repeated cycles of stress that results in self-propping of previously un-propped fractures which increases flow rate and SRV resulting in additional oil recovery above the base EOR method. The invention includes reduction or removal of previously existing formation damage by repeated cycles of C3-C4 as a solvent which increases flow rate and SRV resulting in additional oil recovery above the base EOR method.

BACKGROUND Field of the Invention

The present invention relates generally to Enhanced Oil Recovery (EOR)and more particularly to a method and system for increasing recoveryyield over prior EOR methods.

Description of the Problem Solved

Due to the vast amount of oil and gas trapped in shale reservoirs andextra tight rock reservoirs in the United States and elsewhere, therehas been considerable effort in recent years to extract thesehydrocarbon fluids. Shale is a mineral form that is particularly easy tofracture along various planes. Shale reservoirs contain “rich” field gasthat contains propane (C3) and butane (C4) as well as oil that containsC3 and C4. A typical gas and oil (hydrocarbon) extraction technique forshale is to use methods that fracture the rock at points along ahorizontal well. The hydrocarbon fluids can be extracted from thesefracture points and recovered using natural pressure. The technique iscolloquially known as “fracking”. Extra tight rock reservoirs, includingsandstones or carbonates, behave similarly to shale reservoirs. Afterseveral years, the production from these wells declines as pressuredecreases. Ultimately the wells reach their economic limit whereoperating expenses exceed revenues, at which point the wells must berefraced, redrilled, or abandoned. Methods to combat this naturaldecline in production and increase recovery of the remaining hydrocarbonfluids are known as Enhanced Oil Recovery (EOR).

There are three major prior art methods of EOR: thermal injection,solvent injection and chemical injection. Thermal injection uses hotwater and/or steam to heat the reservoir and cause highly viscous oil tobecome less viscous, flow to the well bore and be pumped to the surface.Solvent injection is a process by which various gases such as carbondioxide, nitrogen, or methane rich gas are injected into the reservoir.These gases are used to reduce oil viscosity, promote miscibility, andincrease reservoir pressure that in turn pushes some of the remainingoil and gas to the producing wells where it is pumped to the surface.Chemical injection involves surfactants, nano-particles, and/or microbesthat employ a variety of mechanisms that release some of the remainingoil and gas. Chemical injection is often used in conjunction with eitherof the other techniques.

Almost all EOR methods in shale and extra tight rock are cyclic (called“Huff-and-Puff”), involving alternating phases of injection followed byproduction from the same wells. During an injection phase (Huff), thesubstance being injected is forced into the well until the pressurereaches a desired maximum in the reservoir which may be between2000-9000 psi. The injection phase is followed by the production phase(Puff), where the hydrocarbon fluids are produced back up the well asthe pressure drops. When the pressure reaches a specified minimum,another Huff and Puff cycle starts. Each Huff phase and Puff phase isvariable and can range from days, to weeks, to months, depending on anumber of variables determined by the reservoir, oil and othercharacteristics.

Sheng in U.S. Published Patent Application 2017/0159416 teachesoptimization techniques for Huff-and-Puff using a gas such as methane(C1), natural gas, carbon dioxide, nitrogen, or combinations thereof.Sheng attempts to optimize the cycle times and pressures.

Valencia et al. in U.S. Published Patent Application 2017/0283688 teachan EOR process that uses an enriched hydrocarbon fluid derived in-situfor injection. Valencia uses C2+(which is primarily ethane) or C3(propane) at reduced temperatures. Enrichment generally is accomplishedby removing C1 (methane), and cooling the injection fluid to −30 degreesF. Valencia also teaches fluid reinjection.

Babcock et al. in U.S. Published Patent Application 2018/0057732 teach amethod of EOR from a resource reservoir that injects fluids via aninjection well and extracts via a nearby production well. The injectionfluid may consist of a mixture of C2 (ethane), C3 (propane), C4 (butane)and C5+(pentane and longer hydrocarbons). The mixture is cooled below 0degrees F. Babcock also teaches using C1 (methane), nitrogen and carbondioxide. Babcock in U.S. Published Patent Application 2018/0058182teaches using Y-Grade Natural Gas Liquids (NGL) in the same mannerdescribed above in this paragraph.

Enhanced Oil Recovery (EOR) as relates to the present invention takesplace in both horizontal and vertical wells in shale and extra tightrock formations that have declined in output. Prior art methods includeHuff and Puff where gas or chemicals are injected into the wells on acyclic basis. The previous methods have several key disadvantages: (1)injecting a gaseous mixture into shale reservoirs requires very highpressures in order to achieve miscibility in the reservoir. Two fluids,such as gas and oil, are “miscible” when they form a single phase afterbeing mixed in any proportion when first brought into contact at a givenpressure and temperature. The compressors and related equipment have tobe specially designed and fabricated which takes a long lead time, andis very expensive; (2) very large quantities of gas are required forHuff and Puff, necessitating additional pipelines capable of handlinghigh pressures running from gas sources to the compressors and then tothe various producing wells. This requirement also adds to the capitalexpenditures (CAPEX). The increased CAPEX and need to find sufficientavailability of the right kind of gas automatically eliminate many oiland gas fields and existing wells from utilizing the previous Huff andPuff methods. Therefore, a Huff-and-Puff method that uses only liquidsinstead of gases alone or gases combined with liquids would havesignificant advantages in certain reservoirs and fields by lowering theCAPEX due to (a) substituting existing, easy to modify pumps to pushhydrocarbon fluids into the formation to stimulate the oil therebyeliminating the need for costly compressors; (b) being able to transportthe hydrocarbon liquids by truck instead of pipeline, eliminating theneed to source gas and lay expensive pipelines and taps; and (c) usingmodular surface equipment that is transportable and has a much smaller“footprint” to accomplish (1) and (2) described above which furtherreduces the CAPEX.

SUMMARY OF THE INVENTION

The present invention relates to Enhanced Oil Recovery (EOR) from aginghorizontal and vertical wells in shale and extra tight rock formations.A mixture of C3 (propane) and C4 (butane) is cyclically pumped as astimulation treatment into the well in liquid phase to “fill” the wellup to a pressure of approximately 3000 psi (although other pressures arewithin the scope of the present invention). The fill portion of thecycle lasts approximately ten to sixty days (although other time periodsare within the scope of the present invention). The well is then allowedto produce for approximately 30 to 120 days (although other time periodsare within the scope of the present invention). Because the entirehydrocarbon fluid is in liquid phase, and since the pressure necessaryto pump the fluids into the formation is relatively low, the stimulationtreatment can be accomplished with a pump rather than a high pressuregas compressor. A “slug” of C3-C4 in liquid phase is loaded from tankertrucks to a holding tank at around 100 psi. The pump delivers the liquidinto the well at a pressure of around 3000 psi. The ratio of C3/C4 canbe adjusted to optimize recovery from given wells in given basins. Theratio can run from 0 (100% C4) to 1 (100% C3), with any intermediateratio available. The injection usually starts with a ratio of 0.5(50-50) but may be adjusted as fluid cost and reservoir characteristicsdemand. As the production phase of the Huff and Puff proceeds, threefluids return from the wellhead: liquid oil, water, and vapor streams.The method of the present invention recovers almost all of thestimulation treatment C3-C4 mixture from the vapor stream and from theoil stream. Water is removed in standard ways known in the art. Theremaining oil and gas are shipped or piped to refineries or other marketdestinations. The recovered C3-C4 is recycled as a liquid stream andused for the next stimulation treatment Huff phase. The system thusbecomes self-sustaining since any of the C3-C4 that is left in thereservoir as part of the process is made up by the additional C3 and C4that is inherent in the newly recovered oil and gas from the reservoir.Because the equipment required to pump liquid C3-C4, and separate theproduced component liquid and vapor streams and recover it is relativelycompact and light, the entire surface apparatus can be mounted on a skidthat can be transported from location to location as needed. Theeconomic model is driven by the exclusive use of liquid components thatare pumped through the wells into the formation instead of first beingcompressed and then injected. The result is to increase the potentialnumber of wells and reservoirs that can benefit from the presentinvention by (a) eliminating custom manufactured high pressurecompressor systems; (b) eliminating the resulting permanent facilitycosts including above or below ground storage capacity for the blendedgas and treatment fluids necessary for the compressors to use, (c)reducing CAPEX by re-deploying the same skid mounted equipment since itis now transportable instead of relatively permanent; (d) eliminatingexpensive pipelines to bring the necessary volumes of gas to theinjection facility; and (e) reducing CAPEX by eliminating the need toadd future gas to mix with the treatment fluids.

In addition to just C3 and C4, other liquid phase products may be addedto the stimulation treatment mixture including C5+(natural gasolinewhich includes pentane C5 and longer hydrocarbons) and C2 (ethane).Again, the ratios can be adjusted to optimize production.

C5+ refers to higher molecular weight hydrocarbons which are liquid atstandard conditions of pressure and temperature including pentane (C5),hexane (C6), heptane (C7), octane (C8), and nonane (C9). The term C5+can refer to any of these pure hydrocarbons (including isomers) in thatmolecular weight range, or any mixture of such hydrocarbons in anycombination.

In an alternate embodiment, pure C4 is pumped into the well. During theproduction phase, this C4 exits at the wellhead mostly dissolved in theoil. Under certain market conditions, this enriched oil will bring ahigher price than other varieties of crude oil. In this embodiment, theenriched oil is sold as-is after normal stabilization.

DESCRIPTION OF THE FIGURES

Attention is now directed to several inline figures:

FIG. 1 shows a simulation of EOR using prior art techniques.

FIG. 2 shows a simulation of EOR using the principles of the presentinvention.

FIG. 3 shows a block diagram of an embodiment of the present inventionbeing used with a group of twenty wells.

FIG. 4 shows a schematic diagram of an embodiment of the presentinvention at a well head.

FIG. 5 shows a block diagram of an embodiment of the skid mountedrecycling unit.

Several figures have been presented to illustrate features of thepresent invention. The scope of the present invention is not limited towhat is shown in the figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention pumps a liquid C3-C4 mixture into a well at arelatively low pressure, with approximately 3000 psi being preferred, asa stimulation treatment. While this is the preferred pressure, differentpressures may be used with different formations at different reservoirdepths. By C3 we mean propane, and by C4 we mean butane (including anyratio of straight chain butane to iso-butane). A pre-mix of liquid C3-C4is loaded from transport vehicles such as a tanker truck into a holdingtank on site. A pump is used to pump the mixture into the well at thedesired pressure which is typically around 3000 psi. A major advantageof the present invention for Huff and Puff EOR is the use of liquidsthat only requires a pump to place them into the formation, instead of ahigh pressure compressor system which is required to inject a vaporstream into wells. The stimulation treatment Huff phase of a given welllasts approximately 10 to 60 days and the production of Puff phase lastsapproximately 30 to 120 days. The total Huff and Puff cycle period lastsapproximately 40 to 180 days. The Huff and Puff cycle is then repeated.During the production phase, C3-C4 is recovered from the liquid oilstream and the hydrocarbon vapor stream leaving the well head. A majoradvantage of the present invention is that most of the liquid C3-C4pumped into the formation in the Huff phase can be recovered during thePuff phase from the hydrocarbon vapor and liquid streams and then pumpedback into the formation in the next Huff phase stimulation treatment.Thus, while a low cost of C3-C4 is an important criteria when initiatingthe first Huff and Puff cycles, the high recovery rate for recycleallows this new Huff and Puff method to potentially be economic inlocations where the cost of the treatment liquids may be higher.

As experience with a given well is gained, the ratio of C3/C4 can beadjusted to optimize production based on a variety of factors includingthe individual liquid component costs. Typically, the ratio starts outat 0.5 with trial adjustments being made during subsequent pumpingperiods. The entire process may run until production finally drops beloweconomically viable levels. In some embodiments of the invention, theratio may be re-adjusted after the passage of time.

The equipment needed to perform the above-described operation is lightand can be made transportable by assembling it on a skid. Since the skidcan be economically transported from site to site, the CAPEX associatedwith building the infrastructure for a fixed facility is eliminated.Moreover, when the hydrocarbons in a well, or groups of wells, can nolonger be economically recovered, or when desired, the equipment can besimply transported to another location.

Turning to FIG. 1, typically produced separator gas (primarily C1) EORfor prior art gas injected wells is shown in a simulation at a basin inthe western U.S. The ascending line shows the prior art enhancedrecovery using high pressure gas compressors. As can be seen, separatorgas EOR produces more oil than the primary; however, the increase inproduction is very slight.

FIG. 2 shows a simulation of the present invention performing thestimulation treatment on the same well. As can be seen, the cumulativeoil produced by liquid C3-C4 EOR is significantly greater thanHuff-and-Puff EOR using methane gas.

FIG. 3 shows a block diagram of the present invention being used with agroup of twenty wells with equipment on two separate skids at a site. Toclarify, in the figures throughout this patent application, the use of“skid” can mean several “skids” working together. The five wells on theleft depict the stimulation treatment Huff phase from day 1 to day 10.The remaining wells are all in the production or Puff phase, eachproducing for 30 days. The stimulation treatment rotates at 10 dayintervals. Approximately 10,000 stb/d (stock tank barrel per day) ofC3-C4 mixture are pumped as a stimulation treatment into the five wellsover the 10 day period. The projected total C3-C4 recovery over the 30day period is 9,900 stb/d reflecting a 99% recovery factor. While thismay be optimistic, greater than 90% recovery is entirely possible. TheC3-C4 is then recycled in a liquid phase for a subsequent stimulationHuff phase.

It should be noted that the plan shown in FIG. 3 is merelyrepresentative of a possible application of the present invention. Anycombination of the number of wells and the arrangement of skids iswithin the scope of the invention. The number of wells being serviced ina cycle can be adjusted according to the accessibility of the wells andthe particular layout of a given field. Any number of skids may be usedsimultaneously, with two skids shown in FIG. 3 as an example.

The skid, if properly constructed, can have a lifetime of at leastfifteen years. Given the projected amount of oil that can beeconomically recovered from a typical well, a skid will probably betransported to a different location at about five years or whenever thewells' oil and gas production reach their economic limit. The number ofyears a skid is used may vary from site to site. Thus, in this example,if a skid lasts fifteen years, it is in service at three differentlocations for five years each. The skid can be recycled and refurbishedat the five year interval. Thus, a group of two skids, over theirfifteen year lifetime, will service approximately sixty wells using eachskid three times.

FIG. 4 shows a schematic diagram of an embodiment of the presentinvention at a well head. A transport vehicle (not shown) delivers aliquid mixture of C3-C4 to a holding tank (not shown). The quantity inthe holding tank can run between 1000 and 4500 barrels of the liquidmixture. A typical truck can deliver around 200-300 barrels of liquidC3-C4. The pressure at this point is approximately 100 psi. The mixtureenters the C3-C4 pump and is pumped into a well head at approximately3000 psi with a temperature of between approximately 40 and 80 degreesF. The pump can be attached to all the wells that will be serviced, suchas the 10 wells per skid shown in FIG. 3. During the production phase,all fluids from the well head are routed to a separation unit atapproximately 215 psi at around 200 degrees F. This three phase mixtureconsists of water, hydrocarbon vapor, and hydrocarbon liquids. In theseparation unit, water is removed from the fluid mixture using methodsknown in the art. The remaining fluid mixture of vapor and liquid isrouted to a compression/cooling system called a “Recycle System” (alsocalled Recycle Treater). Here the vapor is separated into fuel gas,which is mostly C1 with some C2, and C3-C4. The fuel gas is routed offfor sale and the C3-C4 from the vapor phase is liquefied and routed to astorage tank (labeled “C3C4 tank” in FIG. 4). The hydrocarbon liquidfrom the fluid mixture is separated into oil and liquid C3-C4. The oilis routed to a stock tank for piping or shipping as crude oil. Theliquid C3-C4 is routed to the “C3C4 tank” in FIG. 4. The liquid C3-C4 isrecycled and pumped back into the well head for the next stimulationtreatment. The pump can be a high pressure triplex pump with liquidsolvent seals or any other suitable pump. Such pumps are manufactured bycompanies like National Oilwell Varco and others.

FIG. 5 shows a block diagram of an embodiment of the skid mountedrecycling unit. The well head fluids—water, hydrocarbon vapor, andhydrocarbon liquid—enter the separation unit where water and liquid oilis separated from the vapor. A Natural Gas Liquid (NGL) stabilizerperforms the function of reducing the vapor pressure of liquidhydrocarbon condensate to a value ensuring safety during transportationand storage, and performs separation of lighter from heavier products.In the case of the present invention, additional C3 and C4 are removedfrom the light hydrocarbon vapors by refrigeration or cooling. Anylighter products such as C1 and C2 can be removed from the liquid C3-C4in a gas plant stabilizer.

The crude oil is also stabilized by removing C3 and C4 from it, and canbe “sweetened” if necessary by removing impurities such as hydrogensulfide.

The liquid C3-C4 is recycled as has been discussed.

In an alternate embodiment of the present invention, pure C4 can bepumped into the well to serve as a stimulation treatment, and the C4 canbe allowed to remain dissolved in the final oil product. This allows amore valuable oil product to be directly sold.

Different embodiments of the present invention can include one or morecontrol units associated with the skid equipment. In these embodiments,a rugged field computer or PLC can control and monitor the entirepumping-production process at a skid. In some embodiments, a group ofskids in operation can communicate wirelessly with a remote station orwith each other reporting back operational parameters such as pressures,temperatures, flow rates and other parameters. The skid can also becontrolled remotely from the remote station or from a different skid.Typically, this wireless communication is by radio or satellite sincemost sites are in remote areas. Radios can operate on ISM frequencies orspecially licensed frequencies. If the skid happens to be located wherethere is cellular telephone service, communication may also beestablished over a cellular network.

Thermal induced fracturing (TIF) is a well-known result of pumping coolliquids into warm formations. TIF has been studied as an additionalstimulation method for prior hydraulic fractures in shale reservoirs andtight rock. Various embodiments of the process of the present inventionwill pump cool 40 to 80 degrees F. so it forms a liquid state, and thenpump those liquids into reservoirs that are typically 175 to 300 degreesF. This causes an increased flow rate and stimulated rock volume (SRV)for both pumped and produced fluids resulting in additional oil recoveryabove the base EOR method. Increasing SRV is a major benefit to theinvention process by providing more oil filled rock to be contacted bysolvent.

TIF is known to cause secondary fractures perpendicular to the originalhydraulic fractures in the zone near the wellbore where rock stress isincreased by cooling. Repeated cycles of pumped cool liquids will extendthe secondary fracturing deeper into the reservoir on each cycle causingcontinuously increasing well productivity and SRV.

In addition, to thermal stress the repeated cycles of pressure in thisinvention cause rock stress to increase and decrease each cycle. Fieldobservations in shale wells indicate that the zone of improvedproductivity created by hydraulic fracturing with proppant is in aregion approximately 200 feet from the wellbore, but micro seismicobservations show induced fracturing occurs approximately out to 1000feet from the wellbore. In shale formations the proppant staysrelatively close to the wellbore and far distant induced fractures donot get filled with proppant. During primary production, the pressuredrawdown causes stress to increase which in turn closes the far distantfractures that have no proppant. As part of this invention, liquid C3-C4is pumped as a stimulation treatment causing rock pressure to increaserapidly with an associated decrease in rock stress which allows thepreviously far distant fractures to reopen. Repeated stress cycles areknown to cause “Spalling” of the rock, which is a process where rockchips are formed. Kiel in U.S. Published Patent Application 1976/3933205teaches that multiple hydraulic fracturing cycles with increasing anddecreasing pressure will induce Spalling. The Spalling will cause theinduced fractures to be propped open resulting in improved wellproductivity.

The present invention includes self-propping of far distant inducedfractures as a result of rock Spalling caused by pressure and stressincreases and decreases in repeated cycles. The repeated cycles ofpressure will cause far distant induced fractures to stay permanentlyopen resulting in increased well productivity and SRV resulting inadditional oil recovery.

Hydraulic fracturing of shale wells occurs in stages which are spacedapproximately 200 feet apart along a wellbore lateral resulting in about25 stages for a 5000 foot long lateral. Each stage is pumped separatelyin sequence and is unique relative to the local rock and fluids that areencountered. Field observations indicate that the stages do not flowback uniformly with some stronger than others. In many cases some of thestages do not flow at all and this is attributed to some form of“Formation damage”. Formation damage is a technical category ofproduction engineering that can include many types of phenomenon rangingfrom mechanical to chemical blockage of flow. This invention pumpsliquid C3-C4 into shale wells which is classified as a solvent. As partof this invention, repeated cycles of solvent will remove formationdamage from reduced flow or blocked flow fracture stages. The repeatedcycles of solvent will cause previously damaged fracture stages to beimproved over time resulting in increasing well productivity andincreased SRV resulting in additional oil recovery.

The present invention process starts at a reservoir pressure lower thaninitial, after some period of primary production, perhaps as low as 500psi. Liquid C3-C4 is pumped as a stimulation treatment into the well ina liquid phase to “fill” the well up to a pressure of approximately 3000psi (although other pressures are within the scope of the presentinvention). This “fill” phase is relatively expensive in early cycles.Different embodiments of the present invention can include the additionof water, nitrogen or methane dominated field gas to supplement C3-C4plus other liquid phase products in the early cycles of the EOR process.Water, nitrogen, or methane dominated field gas is lower cost fluid thatcan assist in the “fill” portion of early cycles. Continued addition ofWater, nitrogen, or methane dominated field gas is not beneficial inlater cycles as they begin to interfere with oil production. The optimalapplication of supplemental Water, nitrogen, or methane dominated fieldgas to selected early cycles is part of this invention.

A further embodiment of the current invention involves the applicationof one or more well treatment techniques to improve the SRV in the wellsused in the C3-C4 process. These treatment techniques involve both priorart and new inventions, but will prove beneficial when economicallyjustified for each well in a project. These treatment techniques mayinclude diverting agents to focus solvent on underperforming stages, acoiled tubing unit to direct the C3-C4 solvent to specific points alongthe horizontal well's wellbore, the addition of a small amount ofproppant to the C3-C4 solvent used in the first and potentially othercycles to help maintain access to newly added SRV, and the addition of asmall amount of surfactant to the C3-C4 solvent to alter the wettabilityof the reservoir system. Any or all of these techniques can be appliedto this invention in order to increase the productivity and SRV of anindividual well. Increasing SRV is a major benefit to the inventionprocess by providing more oil filled rock to be contacted by solvent.

A further embodiment of the current invention involves the ability toadjust the operating conditions of the Recycle System to allow for themanufacture of a pumpable solvent stream that has a composition designedto be miscible with the reservoir fluids at current reservoir pressuresand temperatures, thereby increasing the efficiency of the improvedrecovery process and increasing the amount of incremental oil recoveryachievable from all stimulation cycles. The pumped solvent compositionis initially designed specifically based on “equation of state” modelingto be first contact miscible with the oil in the target hydrocarbonreservoir at the low pressure condition existing at the end of primaryproduction. However, since pressure and temperature change with time,the Recycle System is adjustable to optimize the composition with time.

A further embodiment of the current invention involves the ability toadjust the operating conditions of the Recycle System to allow for themanufacture of a larger volume of the pumpable solvent stream that has acomposition that is not miscible with the reservoir fluids at currentreservoir pressures and temperatures. However, the larger volume willincrease the amount of incremental oil recovery achievable from allstimulation cycles.

In an alternate embodiment, pure C4 is pumped into the well. During theproduction phase, this C4 exits at the wellhead mostly dissolved in theoil. Under certain market conditions, this C4 enriched oil brings alower price than other varieties of crude oil. In this embodiment, therecycle system is adjusted to capture most of the entrained C4 forrecycle into the next stimulation phase thereby increasing the amount ofstimulant available and thereby increasing the amount of incremental oilthat can be recovered from the project.

A further embodiment of the current invention involves using the RecycleSystem to treat produced fluids from other wells in the area of interesteven though those wells are not being treated with the cyclicstimulation process. Treating the produced fluids from additional wellsincreases the volume of pumpable solvent available for the stimulationof wells in the project, thereby reducing the amount of solvent thatmust be imported for the project and thereby increasing the economicvalue of the project.

A further embodiment of the current invention involves using the RecycleSystem to treat all produced fluids on a given well pad/production sitein order to provide pumpable solvent for stimulation of the reservoirwhile also reducing the fugitive gas emissions that are inherent in mostoil and gas production operations, thereby reducing greenhouse gasemissions associated with the existing operation.

Finally, the present invention may be used in a Greenfield Development.Greenfield Developments typically refer to developing on previouslyundeveloped land. This includes newly discovered oil reservoirs. Hence,a further embodiment of the current invention involves the incorporationof the cyclic stimulation process and the Recycle System in the designof any new development project in existing or newly discovered oilreservoirs deemed susceptible to benefits from the cyclic stimulationsystem, thereby increasing the expected volume of oil to be recoveredfrom the project and reducing the amount of fugitive gas emissionsexpected to be generated from the project with an overall improvement inthe economic outlook for the new development project.

Several descriptions and illustrations have been presented to aid inunderstanding the present invention. One with skill in the art willrealize that numerous changes and variations may be made withoutdeparting from the spirit of the invention. Each of these changes andvariations is within the scope of the present invention.

1-14. (canceled)
 15. A liquid phase cyclic method of enhanced oilrecovery (EOR) from an existing shale or extra-tight rock well, themethod having a pumping phase and a production phase that includespumping pure hydrocarbon liquid propane (C3) and liquid butane (C4) intothe well, the method comprising: (a) receiving a pure hydrocarbon slugof liquid C3 and liquid C4 having a first adjustable ratio of liquid C3to liquid C4 at the existing well to perform enhanced oil recovery; (b)adjusting said first adjustable ratio to form a first pumpable mixturewith a first ratio of liquid C3 to liquid C4; (c) with a pump, pumping aportion of the first pumpable mixture in said first adjustable ratiointo the existing well during the pumping phase at a pressure ofapproximately 3000 psi; (d) recovering pure hydrocarbon liquid C3 andliquid C4 from production oil during a subsequent production phase usinga recycling system to produce a recovered mixture, the recycling systemcomprising at least a crude oil stabilizer; (e) adjusting the ratio ofliquid C3 to liquid C4 in said recovered mixture to form a recycledmixture with a second adjustable ratio of liquid C3 to liquid C4; (f)with the pump, pumping a portion of the recycled mixture into the wellduring a subsequent pumping phase at a pressure of approximately 3000psi; (g) repeating steps (d)-(f) in subsequent production andstimulation cycles; (h) mounting at least one of the pump or the crudeoil stabilizer on a skid.
 16. The method of claim 15, wherein acompressor or a refrigeration or a cooling unit is also mounted on theskid; the compressor or refrigeration or cooling unit configured toproduce additional pure hydrocarbon liquid C3 and liquid C4 to be addedto the recovered mixture from production vapors. 17-18. (canceled) 19.The method of claim 15, further comprising using, at a group of wells, aplurality of skids, each of the plurality of skids located at aparticular well of the group of wells, each skid containing componentsof the recycling system configured to process fluids and vapors producedfrom the particular wells.
 20. The method of claim 15, wherein theexisting well has a first temperature, and the first pumpable mixture,or the recycled mixture has a second temperature, the second temperaturebeing less than half of the first temperature.
 21. The method of claim20, wherein the second temperature is between approximately 40 and 80degrees F., and the first temperature is approximately 175 to 300degrees F.
 22. (canceled)
 23. The method of claim 15, further comprisingadjusting the first and second adjustable ratios of liquid C3 to liquidC4 to allow the pumpable mixture to be miscible with reservoir fluids atcurrent reservoir pressures and temperatures.
 24. The method of claim15, wherein the first and second adjustable ratios of liquid C3 toliquid C4 in the recycled mixture are changed over time to optimizeproduction.
 25. The method of claim 15, wherein operating conditions ofthe recycling system allow manufacture of an increased volume of therecycled mixture having a composition that is not miscible withreservoir fluids at current reservoir pressures and temperatures. 26.The method of claim 15, wherein, the recycling system treats producedfluids from other wells in an area of interest that are not beingtreated with the liquid phase cyclic method to increase volume of saidpumpable mixture available for wells being treated with the liquid phasecyclic method.
 27. A liquid phase cyclic method of enhanced oil recovery(EOR) from an aging existing shale or extra tight rock well, the methodhaving a pumping phase and a production phase resulting in additionaloil recovery comprising: with a pump, pumping into the aging existingshale or extra tight rock well a treatment fluid mixture of purehydrocarbon liquid propane (C3) and liquid butane (C4) in a firstadjustable ratio as a liquid phase solvent during the pumping phase;recovering pure hydrocarbon liquid C3 and liquid C4, in a secondadjustable ratio, from production products during the production phasethrough deployment of a skid-mounted recycle system comprising at leasta crude oil stabilizer, mounted on a skid; recycling said purehydrocarbon liquid C3 and C4 as a recycled mixture with the pump intothe well in a subsequent pumping phase.
 28. The method of claim 27wherein the skid-mounted recycling system further includes a compressoror a refrigeration unit or a cooling unit.
 29. The method of claim 27,wherein additional make-up liquid C3 or liquid C4 is added to therecycled mixture.
 30. (canceled)
 31. The method of claim 27, furthercomprising using hydrocarbon vapors and liquids supplied from oil andgas production operations outside an EOR project and processed throughthe skid-mounted recycle system to generate said recycled mixture. 32.The method of claim 27, further comprising treating produced fluids withthe skid-mounted recycle system on a particular EOR well pad orproduction operation outside an EOR project to reduce fugitive gasemissions.
 33. The method of claim 27, further comprising deploying theskid-mounted recycle system in a greenfield development wherein fugitivegas emissions are reduced, and the treatment fluid mixture is availableto increase oil production using the liquid phase cyclic method ofenhanced oil recovery.
 34. The method of claim 27 producing a stabilizedcrude oil product, further comprising rejecting aromatic hydrocarbonsfrom the fluid treatment mixture into a stabilized crude oil product toprevent said aromatic hydrocarbons from being pumped back into thereservoir.
 35. A method of enhanced oil recovery (EOR) from an agingexisting shale or extra tight rock well having alternating pumpingphases and a production phases comprising: (a) mounting a crude oilstabilizer and a pump on a skid; (b) locating the skid near an existinghorizontal or vertical aging existing shale, or existing extra tightrock well; (c) storing a pure hydrocarbon mixture of liquid C3 andliquid C4 having an adjustable ratio of liquid C3 to liquid C4 near theskid; (d) adjusting said adjustable ratio to form a first pumpablemixture with an initial ratio of liquid C3 to liquid C4; (e) with thepump, pumping a portion of the first pumpable mixture in said initialratio into the existing well during an n th pumping phase, where n is apositive integer; (f) recovering pure hydrocarbon liquid C3 and liquidC4 from production oil during the nth production phase using a recyclingsystem to produce a recovered mixture, the recycling system using atleast the crude oil stabilizer; (g) adjusting the adjustable ratio ofliquid C3 to liquid C4 in said recovered mixture to form a recycledmixture with a second ratio of pure hydrocarbon liquid C3 to liquid C4;(h) pumping a portion of the recycled mixture into the well during ann+1 th pumping phase; (i) increasing n by 1; (j) repeating steps (f)-(j)until the well no longer produces an economical quantity of oil.
 36. Themethod of claim 15, wherein a compressor or a refrigeration or a coolingunit is also mounted on the skid; the compressor or refrigeration orcooling unit configured to produce additional pure hydrocarbon liquid C3and liquid C4 from production vapors to be added to the recoveredmixture.
 37. The method of claim 15, wherein C5+ is added to therecycled mixture at step (g).